1. Field of the Invention
This invention relates to a process for the selective concentration of metal values present in solution. In particular, the present invention employs a multiple-phase system having a common solvent and the preferential affinity of the metal values for one of the phases of said multiple-phase system. A metal-containing phase may thereafter be isolated and the metal values recovered therefrom, allowing for the recycle of the liquid from the isolated phase for reestablishing a multiple-phase system.
2. Prior Art
In the recent past, technology relating to the recovery of metal values has received increased attention. This attention has focused on processes varying from the refining of raw ores to the removal of trace amounts of metals from industrial waste streams. In the area of ore refining, this recent interest is due in part to the increasing scarcity of high grade ores and the attractive prices which refined metals command. Environmental concerns stemming from the release of metal-containing industrial effluent streams have undoubtedly contributed to the second area of research.
The refining of crude ores into high purity metals frequently involves a process, commonly known as solvent extraction, wherein an aqueous leaching solution is first contacted with the ore, thereby producing an aqueous metal-containing solution. This is followed by the establishment of a two-phase system through addition of a diluent comprising a water-immiscible hydrocarbonaceous liquid. Extractants are then typically added to aid in the migration of selected metal values to the hydrocarbon phase in which they are preferentially soluble. The selected metals are thereby concentrated in the hydrocarbon phase which can then be decanted and the selected metals removed therefrom, typically into a virgin aqueous phase through a process known as stripping. The metal values may then be recovered from the aqueous phase by conventional methods such as sedimentation, formation of metal hydroxide precipitates and subsequent filtration of the metal-containing precipitates, crystallization or electrolytic reduction.
The use of solvent extraction in the isolation of material from an aqueous solution was disclosed as early as 1903 in U.S. Pat. No. 744,795. This reference discloses the use of aliphatic or aromatic hydrocarbonaceous solvents in the establishment of the two-phase systems. The use of extractants such 8-hydroxyquinone, dithizone and phenanthroline was also disclosed.
with the emergence of the nuclear industry, commercial application of solvent extraction technology flourished. For example, U.S. Pat. No. 2,564,241 discloses the concentration of uranium from its ores through solvent extraction employing alkylam-ne extractants. Other commonly used extractants employed in the refinement of uranium are di(2-ethylhexyl) phosphoric acid as well as secondary, tertiary and quaternary amines.
In addition to the refining of uranium, solvent extraction has been proposed for the isolation of many metals from their ores and is currently commercially employed in the refining of copper, nickel, cobalt, chromium, boron, calcium, vanadium, molybdenum, tungsten, zinc, gold, rare earths, platinum group metals, tantalum, niobium, zirconium and hafnium. For example, boron may be recovered from brines through solvent extraction with water-insoluble organic diluents, such as kerosene, containing from 0.8 to 15.0 percent volume of hydroxy oximes, as disclosed in U.S. Pat. No. 4,324,771.
However. the refining of metal-containing ores through the use of solvent extraction is not without problems. The majority of these problems stem from the use of the organic solvent itself due to its relatively high initial cost, the environmental, fire and health hazards which it typically poses, as well as the economic costs associated with its containment.
In response to these concerns, refining techniques wherein no organic solvent is employed have been proposed. For example U.S. Pat. No. 4,321,089 discloses a process for the recovery of molybdenum and rhenium metals from their sulfide ores which involves the application of microwave energy to the ore material in the presence of oxygen or chlorine, followed by recovery of the metals from the respective metal oxides and chlorides. While avoiding the solvent-imposed toxicity problems. this process can be readily seen to be both energy and capital intensive, as well as raising its own environmental concerns due to the use of microwave radiation and chlorine gas.
As previously mentioned, metal recovery in areas other than ore refinement has also received increased attention in the recent past. For example, due to the environmental and health concerns now being attributed to the release and contact with heavy metals such as lead, mercury, arsenic, chromium, cadmium and vanadium, the removal of these materials from industrial effluent streams has been the focus of recent research. For example, U.S. Pat. No. 3,769,205 discloses a process for the recovery of mercury from effluent streams through a solvent extraction process employing mixtures of organic sulfides or disulfides. U.S. Pat. No. 3,856,917 discloses a process whereby chromium may be recovered from aqueous waste streams containing hexavalent chromium through solvent extraction followed by extraction of the chromium-containing hydrocarbon phase with an aqueous sodium chromate solution. The chromium may then be recovered as sodium bichromate. U.S. Pat. No. 4,349,514 discloses a process for the recovery of chromic acid from aqueous waste streams through electro-dialysis, sulfuric acid addition and solvent extraction. U.S. Pat. No. 4,425,236 discloses a method for the recovery of cadmium from cadmium-containing aqueous waste streams through solvent extraction employing water-insoluble organic polysulfide diluents. U.S. Pat. Nos. 3,258,307 discloses a method for the separation of cadmium and zinc from an aqueous solution of their sulfates employing an extraction using an organic quaternary ammonium halide diluent. U.S. Pat. No. 4,317,804 discloses the removal of ferric ion from aqueous streams through solvent extraction using a C.sub.6-12 dialkyl phosphoric acid extractant in a water-insoluble hydrocarbonaceous diluent.
However, all of the above processes employ solvent extraction techniques which again impose their own toxicity problems as well as the high costs associated with their minimization.
Economic considerations have further dictated that methods be developed for the recovery of valuable metals such as gold, silver, platinum and tungsten from media other than virgin ores. For example, silver is often reclaimed from silver-containing photographic waste material prior to its disposal. Silver and platinum are also routinely reclaimed from spent catalytic materials. Copper is further present in streams produced during the etching of printed circuit boards in concentrations sufficient to warrant its recovery. Chromium is present in sufficient concentrations in effluent streams produced during the tanning of leather to call for its recovery based upon economic as well as environmental considerations. However, the methods employed in the recovery of these metal values typically involve solvent extraction techniques and therefore continue to suffer from the drawbacks described above.
Techniques for the recovery of these materials employing methods other than solvent extraction have therefore also been developed. For instance, U.S. Pat. No. 4,332,584 discloses a process for the recovery and recycle of chromium values produced during the tanning of leather wherein a chromium-containing stream is de-watered and pyrolyzed followed by removal of the chromium from the residue by leaching with sulfuric acid. It is readily apparent that this alternative to solvent extraction is energy intensive and employs substances which simply raise different toxicological and environmental concerns. U.S. Pat. Nos. 3,829,549, 3,982,932 and 4,135,976 all relate to the recovery of silver from photographic processing solutions through the use of bacteria or bacterially-produced enzymes. While succeeding in avoiding the toxicological problems associated with solvent usage, these processes instead suffer costly operating constraints such as maintenance of proper pH, temperature and oxygen content of the liquid media. Furthermore, these processes require subsequent separation of the silver-containing compounds from the resulting bacterial population, thereby necessitating further process steps and the additional equipment. U.S. Pat. No. 4,428,773 discloses a process for the recovery of copper values from spent etching solutions produced in the manufacture of printed circuit boards. This process involves contacting the etchant solution with formaldehyde in titanium vessels under an oxygen-containing atmosphere at a pH of between 12 and 14 and temperatures of from 200.degree.-210.degree. F. This process can also be seen to employ toxic reactants as well to require severe operating conditions and high capital investment in equipment.
Affinity partitioning has been employed in the separation of biological material and involves the establishment of multiple, distinct phases in a common solvent through the addition of substances such as polymeric materials, which produce immiscible phases and the selective affinity of a molecule of biological material for one phase over another. Aqueous two-phase systems have been known since the late nineteenth century from the work of Beijerinck, who initially disclosed the formation of aqueous bi-phase systems composed of agar and gelatin.
Affinity partitioning has offered the biotechnology industry the potential of improved recovery of cellular components, such as enzymes, in applications on an industrial scale as this procedure is not dependent upon particle size as are conventional techniques such as filtration and centrifugation. Furthermore, affinity partitioning offers the additional benefit of allowing the use of systems wherein water is the only solvent present, thereby avoiding such problems as denaturation of the biological material through exposure to non-aqueous media.
Examples of the formation of aqueous multiple-phase systems and their use in the isolation and recovery of biological material include U.S. Pat. No. 4,144,130 which discloses its use in the isolation of enzymes from other cellular matter. U.S. Pat. No. 4,343,735 discloses the isolation of interferon through the use of affinity partitioning, while U.S. Pat. No. 4,207,200 discloses its use in the recovery of deoxyribonucleic acid. The isolation of human coagulation factors VII and VIIa through the use of this technology is further disclosed in U.S. Pat. No. 4,470,969. The use of affinity partitioning has further been utilized in the isolation of enzymes and production of enzyme-containing detergent compositions as disclosed in Applicants' co-pending applications U.S. application Ser. Nos. 728,241, 728,242, and 728,243.
However, as stated above, this technology has not been taught or suggested for use in the recovery of metal values from solution.
In view of the above discussed inadequacies associated with the use of solvent extraction techniques, there is a need for a method for the separation and concentration of metal values present in solution which does not employ a toxic organic phase and therefore does not present a health or ecological hazard. Furthermore, there is a need for such a method which is neither energy nor capital intensive.
It is therefore an object of the present invention to provide a process for the separation or concentration of metals which does not employ the toxic materials commonly used in solvent extraction.
It is further an object of the present invention to provide a process for the separation or concentration of metals which is neither capital nor energy intensive.
It is still another object of the present invention to provide a process for the separation or concentration of selected metals from solutions containing mixtures of metals.